In air-conditioning apparatuses, R410A which is a HFC refrigerant has thus far been mainly employed as refrigerant to circulate in a refrigerant circuit. Although R410A has zero ozone layer depletion potential (ODP) and hence makes no harm to the ozone layer unlike conventional HCFC refrigerants such as R22, R410A has high global warming potential (GWP). Accordingly, the HFC refrigerant having high GWP such as R410A is being gradually replaced with other HFC refrigerants having low GWP, from the viewpoint of global warming prevention.
Examples of the HFC refrigerants having low GWP include halogenated hydrocarbon containing a carbon double bond in the composition, typically exemplified by HFO-1234yf (CF3CF═CH2; tetrafluoropropane), HFO-1234ze (CF3—CH═CHF), and HFO-1123 (CF2═CHF). Although those refrigerants belong to the group of HFC refrigerants, these refrigerants are often called HFO, where O represents olefin, because unsaturated hydrocarbon containing a carbon double bond is called olefin. Therefore, the refrigerants described above will be denoted as HFO refrigerants, for distinction from the HFC refrigerants that do not contain a carbon double bond in the composition, such as R32 (CH2F2; difluoromethane) and R125 (CHF2—CF3; pentafluoroethane) contained in R410A.
Although the HFO refrigerant having low GWP may be employed in the form of a single refrigerant, the HFO refrigerant may also be employed in the form of a mixed refrigerant containing a plurality of types of other refrigerants including a HFC refrigerant such as R32. Although the HFO refrigerant, or a mixed refrigerant containing the HFO refrigerant and the HFC refrigerant is not as highly flammable as a HC refrigerant such as R290 (C3H8; propane), such refrigerant is slightly flammable unlike R410A which is non-flammable. Accordingly, refrigerant leakage has to be taken care of. The refrigerants that have flammability, from low flammable ones to highly flammable ones, will hereinafter be called flammable refrigerants. R32 exhibits low flammability like HFO refrigerants, when used in the form of a single refrigerant. Therefore R32 is a flammable refrigerant, and consequently the mixed refrigerant containing the HFO refrigerant and R32 is also regarded as a flammable refrigerant. However, R410A which is a mixture of R32 and R125 is non-flammable owing to the characteristic of R125.
In case that such flammable refrigerant leaks out into room air and resides in the room without diffusing, a gas phase of flammable concentration may be generated in the room, and therefore in case that an ignition source is present in the gas phase of flammable concentration, the refrigerant may catch fire. As first step for avoiding combustion of the refrigerant which has leaked out into the room, the refrigerant leakage has to be detected by the air-conditioning apparatus.